1. Field of the Invention
The invention relates to a soot sensor configured to detect soot particles in an exhaust gas flow having a structure comprising measurement electrodes engaging interdigitally with one another, which are applied on a substrate, the electrical resistance between the measurement electrodes being measured as a measure of a soot load of the exhaust gas flow.
2. Description of the Related Art
The buildup of pollutants from exhaust gases in the atmosphere is a topic of great discussion at present. Associated with this is the fact that the availability of fossil energy sources is limited. In response to this combustion processes in internal combustion engines are being thermodynamically optimized to improve their efficiency. In the automotive sector, this is reflected in the increasing use of diesel engines. The disadvantage of this combustion technology in relation to optimized gasoline engines, however, is a significantly increased output of soot. Soot is particularly carcinogenic due to the accumulation of polycyclic aromatics, which has already been responded to in various regulations. For example, exhaust gas emission standards with maximum limits for soot emission have been established. There is therefore a need for economical sensors which reliably measure the soot content in the exhaust gas flow of motor vehicles.
Soot sensors are used to measure the amount of soot instantaneously being output, so that the engine management in an automobile can receive information for a current driving situation to reduce the emission values by control technology adaptations. Furthermore, with the aid of soot sensors, active exhaust gas cleaning by exhaust gas soot filters can be initiated or exhaust gas may be recycled to the internal combustion engine. In the case of soot filtering, regenerable filters are used that filter a substantial part of the soot content from the exhaust gas. Soot sensors are required for the detection of soot, to monitor the function of the soot filters or control their regeneration cycles.
To this end, a soot sensor may be connected before and/or after the soot filter, which is also referred to as a diesel particle filter.
The sensor connected before the diesel particle filter is used to increase the system reliability and ensure operation of the diesel particle filter under optimal conditions. Since this depends to a great extent on the mass of soot accumulated in the diesel particle filter, it is very important to have an accurate measurement of the particle concentration before the diesel particle filter system, particularly in order to determine a high particle concentration before the diesel particle filter.
A sensor connected after the diesel particle filter offers the opportunity to carry out an on-board diagnosis, and furthermore serves to ensure optimal operation of the exhaust gas after-treatment system.
There have been various approaches for the detection of soot in the prior art. One approach that has been followed extensively in the laboratory is the use of light scattering by the soot particles. This procedure is very suitable for elaborate measuring instruments. When attempting to use them as a mobile sensor system in the exhaust gas system as well, it should be mentioned that these approaches for producing an optical sensor in a motor vehicle entail very high costs. Furthermore, unresolved problems remain in relation to the required optical windows being polluted by combustion exhaust gases.
Laid-open German patent specification DE 199 59 871 A1 discloses a sensor and an operating method for the sensor, both of which being based on thermal observations. The sensor comprises of an open porous shaped body, for example a honeycomb ceramic, a heating element and a temperature sensor. When the sensor is brought in contact with a volume of gas to be measured, soot is deposited on it. For the measurement, the soot deposited in a period of time is ignited with the aid of the heating element and burnt. The temperature increase resulting from the combustion is measured.
Particle sensors for conductive particles are currently known, in which two or more metal electrodes are provided which comprise measurement electrodes engaging in one another in a comb-like fashion, or interdigitally. Soot particles, which are deposited on these sensor structures, short circuit the measurement electrodes and therefore change the impedance of the electrode structure. With an increasing particle concentration on the sensor surface, a decreasing resistance, or an increasing current with a constant voltage applied between the measurement electrodes, can thereby be measured. Such a soot sensor is disclosed, for example, in DE 10 2004 028 997 A1.
The comb-like (interdigital) measurement electrode structure of these soot sensors is generally formed by thin conductor tracks lying next to one another. The conductor tracks have, for example, a spacing of 10 μm from one another. Besides the desired resistance change of the soot sensor due to soot loading of the comb-like structure consisting of measurement electrodes, the resistance of the soot sensor may also change owing to undesired short circuits. These undesired short circuits may be caused by a scratched or partially detached measurement electrode. The measured resistance value of the soot sensor would be vitiated by these undesired short circuits, which must then be established by regular function diagnosis of the soot sensor in order to comply with legal requirements in force for the reliability of exhaust gas after-treatment systems. Furthermore, the resistance values may be influenced by the substrate itself, on which the measurement electrodes are arranged. Above all, when the substrate is heated, currents can flow through the hot substrate from one measurement electrode to another. Undesired deposits, for example ash, may also allow currents that vitiate the measurement of the soot load to flow between the measurement electrodes.